1. Field of the Invention
The present invention relates to a thin-film photoelectric conversion device, especially a solar cell which is formed on a substrate. and more particularly to a thin-film solar cell having a photoelectric conversion layer formed of a crystalline silicon film.
2. Description of the Related Art
A solar cell or a solar battery can be manufactured using a variety of semiconductor materials or organic compound materials. However, from the industrial viewpoint, silicon which is of a semiconductor is mainly used for the solar cell. The solar cells using silicon can be classified into a bulk solar cell using a wafer of monocrystal silicon or polycrystal silicon and a thin-film solar cell having a silicon film formed on a substrate. The reduction of manufacture costs is required, and the thin-film solar cell is expected in the effect of reducing the costs because raw materials are less used for the thin-film solar cell than that for the bulk solar cell.
In the field of thin-film solar cell, an amorphous silicon solar cell has been put into practical use. However, since the amorphous silicon solar cell is lower in conversion efficiency compared with the monocrystal silicon or polycrystal silicon solar cell and also suffers from problems such as the deterioration due to light and so on the use thereof is limited. For that reason, as another means, a thin-film solar cell using a crystalline silicon film has been also developed.
A melt recrystallization method and a solid-phase growth method are used for obtaining a crystalline silicon film in the thin-film solar cell. Both the methods are that an amorphous silicon is formed on a substrate and recrystallized, thereby obtaining a crystalline silicon film. In any event, the substrate is required to withstand the crystallization temperature, whereby usable material is limited. In particular, in the melt recrystallization method, the substrate has been limited to a material that withstands 1,412.degree. C., which is the melting point of silicon.
The solid-phase growth method is of a method in which an amorphous silicon film is formed on the substrate and crystallized thereafter through a heat treatment. In such a solid-phase growth method, in general, as the temperature becomes high, the processing time may be shortened more. However, the amorphous silicon film has been hardly crystallized at a temperature of 500.degree. C. or lower. For example, when the amorphous silicon film which has been grown through a gas-phase growth method is heated at 600.degree. C. so as to be crystallized, 10 hours are required. Also, when the heat treatment is conducted at the temperature of 550.degree. C., a 100 hour or longer is required for the heat treatment.
For the above reason, a high heat resistance has been required for the substrate of the thin-film solar cell. Therefore, glass, carbon or ceramic was used for the substrate. However from the viewpoint of reducing the costs of the solar cell those substrates are not always proper and it has been desired that the solar cell is fabricated on a substrate which is most generally used and inexpensive. However for example the #7059 glass substrate made by Corning, which is generally used has a strain point of 593.degree. C., and the conventional crystallization technique allows the substrate to be strained and largely deformed. For that reason, such a substrate could not be used. Also, since a substrate made of a material essentially different from silicon is used, monocrystal cannot be obtained even through crystallization is conducted on the amorphous silicon film through the above means, and silicon having large crystal grains is hard to obtain. Consequently, this causes a limit to an improvement in the efficiency of the solar cell.
In order to solve the above problems, a method of crystallizing an amorphous silicon film through a heat treatment is disclosed in U.S. Pat. No. 5,403,772. According to the method disclosed in this patent, in order to accelerate crystallization at a low temperature, a small amount of metal elements are added to the amorphous silicon film as a catalyst material. Further, there is disclosed that the lowering of a heat treatment temperature and the reduction of a treatment time are enabled. Also, there is disclosed in the publication that a simple substance of nickel (Ni), iron (Fe), cobalt (Co) or platinum (Pt). or a compound of any one of those materials and silicon or the like is suitable for the catalyst material.
However, since any catalyst material used for accelerating crystallization is a material which is naturally undesirable for crystalline silicon it has been desired that the concentration of the catalyst material is as low as possible. The concentration of a catalyst material necessary for accelerating crystallization was 1.times.10.sup.17 /cm.sup.3 to 1.times.10.sup.20 /cm.sup.3. However, even when the concentration is relatively low, since the above catalyst material is a heavy metal element, the material contained in silicon forms a defect level, thereby lowering the characteristic of a fabricated element.
By the way, the principle of operation of a solar cell fabricated by forming a p-n junction can be roughly described as follows. The solar cell absorbs light and generates the charges of electrons/holes due to an absorbed light energy. The electrons move toward an n-layer side, and the holes move toward a p-layer side due to the drifts caused by a junction electric field and diffusion. However, when the defect level are high in silicon, the charges are trapped by the defect level while they are moving in the silicon, thereby disappearing. In other words, the photoelectric conversion characteristics are caused to lower. A period of time since the electrons/holes generate until they disappear is called "a life time". In the solar cell, it is desirable that the lifetime is long. Hence it has been necessary to reduce the heavy metal element that generate the defect level in silicon as much as possible.